The vasculature plays a principal role in health and disease, by establishing stereotypical vessel organization to ensure proper tissue perfusion. Arteries carry blood from the heart to capillaries, successively reducing vessel caliber. Capillaries, the arterial-venous (AV) interface where exchange of nutrients and waste with tissues occurs, are by necessity the smallest diameter vessels. Post-capillary venules join successively wider veins to return blood to the heart. During development, Notch signaling has emerged as a critical mediator of AV specification during assembly of the first artery/vein pair from endothelial cells (ECs), by promoting arterial over venous EC fate. Notch gene expression is maintained in postnatal vascular endothelium, suggesting that Notch has a role in establishing and/or maintaining postnatal AV organization. Shortly after birth, the immature brain vasculature undergoes a dynamic period of morphogenesis, during which a refined organization of arteries/capillaries/veins forms from a primitive plexus of wide-bore AV connections. Currently, little is known about the genetic controls to ensure such AV organization in the brain. We hypothesize that Notch signaling governs AV organization in the immature mammalian cerebrovasculature, by dictating the allocation of individual ECs from immature AV connections to artery vs. vein. In Aim 1, we hypothesize that individual ECs within the immature brain vascular plexus are allocated to artery vs. vein, thereby permitting vessel narrowing at the AV interface. Toward a comprehensive analysis of AV organization within the brain, we will: 1) examine, over time, AV marker expression in ECs within immature AV connections; 2) monitor EC migration in vivo. In Aim 2, we hypothesize that loss of Notch promotes a venous program and increases EC allocation to the vein from immature AV connections in the brain. We will abrogate Notch signaling specifically in immature ECs and analyze the phenotypic consequences, as relate to AV marker expression and EC migration in the cerebrovasculature. In Aim 3, we hypothesize that activated Notch promotes an arterial program and increases EC allocation to the artery from immature AV connections in the brain. We will activate Notch signaling specifically in immature vascular endothelium and assess phenotypic effects, as relate to AV marker expression and EC allocation to artery vs. vein. During this dynamic period of vessel narrowing and circuitry formation in the immature brain, the vasculature is particularly susceptible to aberrations and associated neurological impairments, underscoring the necessity for proper AV connectivity. These studies will provide molecular and cellular resolution toward the current view of postnatal cerebrovascular morphogenesis and open avenues for therapeutic application.